1. Field of the Invention
This invention relates to a semiconductor device and more particularly to a semiconductor device in which elements are isolated by a local-oxidized (LOCOS) insulating film. This invention also relates to a method for producing the semiconductor device.
2. Description of the Prior Art
Semiconductor devices in which transistors and other elements are formed on one semiconductor substrate generally have a plurality of active regions on which these elements are formed and an isolation region surrounding the active regions on the semiconductor substrate.
Local oxidation (LOCOS) is widely used as an element isolation technology because of its easy production and low production cost.
To simplify the explanation in this specification, insulating films formed by a LOCOS method are referred to as LOCOS insulating films and the oxidation for forming the LOCOS insulating film is referred to as the LOCOS process.
Below is an explanation of the prior art, referring to FIGS. 7A and 7B.
First, an oxide film 62 (50 nm thick) as a protective film and a CVD-Si.sub.3 N.sub.4 film 63 (120 nm thick) as a mask layer are successively grown on a silicon substrate 61, and then a resist 64 is formed on the CVD-Si.sub.3 N.sub.4 film 63 in a pattern which defines the isolation regions on the silicon substrate 61. The resist 64 has openings over the isolation regions on the silicon substrate 61. The CVD-Si.sub.3 N.sub.4 film 63 under these openings is removed by anisotropic etching (see FIG. 7A).
After the resist 64 is removed, the silicon substrate 61 is selectively oxidized in the regions where the CVD-Si.sub.3 N.sub.4 film 63 is removed (see FIG. 7B). In this manner, the LOCOS insulating film 65 is formed in the isolation regions of the silicon substrate 61. The CVD-Si.sub.3 N.sub.4 film 63 in the regions (active regions) where the LOCOS insulating film 65 was not grown on the silicon substrate 61 is removed in a later step, and transistors and other elements are formed in these active regions.
The above prior art technology presents the following problems.
As shown in FIG. 7B, the LOCOS insulating film 65 grows horizontally as well as vertically during the LOCOS process. This horizontal growth can cause the LOCOS insulating film 65 to penetrate under the CVD-Si.sub.3 N.sub.4 film 63. This penetrating part of the LOCOS insulating film 65 is referred to as "bird's beak," and the length of horizontal penetration (bird's beak length) increases as the LOCOS insulting film 65 is made thicker.
Due to this bird's beak, the actual LOCOS insulating film 65 are wider than the designed isolation region, and the actual active regions narrower than the designed active regions.
As the number of active regions increases and their widths become smaller with increased levels of integration, the reduced effective area of active regions due to bird's beak becomes a problem. Particularly when bird's beak becomes longer than half the width of the active regions, the active regions become entirely covered by bird's beak and elements can no longer be formed on those active regions. Even if the active regions are not entirely covered by bird's beak, if they become too small, the characteristics of the MOS field-effect transistors (MOSFET) formed on those active regions is subject to the narrow channel effect.
To prevent this kind of problem, the thickness of the LOCOS insulating film can be reduced to shorten the length of bird's beak, but if the LOCOS insulating film is too thin, the serious problem of insufficient element isolation results.
As described above, when the LOCOS insulting film is made thin to inhibit the penetration of bird's beak in the prior art, the element isolation characteristic is degraded, and if the LOCOS insulting film is made thicker to improve the element isolation characteristics, the MOSFET formed on the small active regions are subject to the narrow channel effect.